Polylactic acid (PL A) is a biodegradable plastic with significantly low potential for causing environmental hazards. Therefore, it has found a wide range of applications including agricultural products, architectural sheets, food wrappings, hygiene materials, fishing nets, fishing lines and various other purposes. It is also widely used for monoaxially and biaxially stretched films, fibers, extrusion products and various other purposes.
Lactic acid has a chiral center and occurs as D-(−) and L-(+) enantiomers. Enantiomeric purity is important for industrial applications and the greatest demand is for the Lisomer. Deliberate blending of the enantiomers provides an effective method to control both the physical properties of polylactic acid and the rate of biodegradation. Lactides are dimeric cyclic esters of lactic acid and are the intermediates in the preparation process of high molecular weight polylactides which are truly biodegradable polymers. The optical purity of L(+) lactide is important especially for the preparation of high molecular weight polylactides for the preparation of films and fibers.
Generally, lactic acid is oligomerized and then catalytically dimerized to make the cyclic lactide .monomer. PLA of high molecular weight is produced from the lactide monomer by ring-opening polymerization using most commonly a stannous octoate catalyst or tin (II) chloride. This mechanism does not generate additional water, and hence, a wide range of molecular weights are accessible. Zinc is also a commonly employed catalyst for such reactions. These processes typically lead to PLA in the range of 60-92% with maximum optical purity obtained up to 99.6%.
U.S. Pat. No. 5,053,522 titled, “Process for the preparation of lactide” discloses continuous or semi continuous process for preparing L(−) or D(+)-lactide of substantial optical purity, wherein L(−)-polylactic acid is employed as the starting material to make L(−) lactide and D(+)-polylactic acid is employed as the starting material to make D(+)-lactide. The starting materials used are of 90%-optical purity and wherein the polylactic acid is—heated to about 130° C. to 230° C. under reduced pressure, in the presence of about 0.05 to 1.0 per cent by weight of a catalyst selected from the group consisting of tin dust, tin halide and organic tin compounds derived from C1-C20 carboxylic acids. L-lactide thus obtained has 99% optical purity and yield in the range of 64 to 69%.
JP Publication No. 2000-015107 titled, “Lactidation Catalyst and Preparation of Lactide” by Shimadzu Corp, published on 18.01.2000, discloses a lactic acid oligomer depolymerized by heating under reduced pressure in the presence of a catalyst containing a zinc compound, represented by the formula Zn(OCOR11)(OCOR12) (I) or ZnX1X2 (II) to prepare lactide. The optical purity of the lactide thus obtained is 99.66% and yield is 92%.
U.S. Pat. No. 3,322,791 titled “Preparation of Optically Active Lactides” discloses a process for preparation of substantially pure optically active lactides from optically active lactic acids, wherein, L(+) lactic acid is converted to LH-lactide and D(−)-lactic acid is converted to D(+) lactide, by heating in presence of 0.1-5 weight percent titanium alkoxide.
U.S. Pat. No. 5,023,349 discloses a gas-assisted continuous process for the rapid conversion of oligomers of alpha-hydroxycarboxylic acids, esters or salts thereof to cyclic esters, L-lactide. The oligomer in the reaction zone contains a catalyst (Sn as the metal (powdered)) effective to depolymerize the oligomer to cyclic ester, the catalyst being present in a catalytically effective amount. L-lactide of high purity in high yield is obtained.
However, the said process has its limitations in view of the fact that it is imperative to maintain the flow rate of the gas which should be sufficiently high. If the flow rate is too low, the conversion to cyclic ester is adversely affected leading to lower yields of L-lactide.
Lactide is generally synthesized by the distillation method which uses polylactic acid with a relatively-low molecular weight called “prepolymer” as an intermediate and which comprises the steps of cyclizing this intermediate at a temperature of 180 to 220° C. in the presence of a catalyst thereby forming lactide which is a cyclic ester formed from two molecules of lactic acid and extracting this lactide in the form of vapor out of the reaction system. The lactide vapor expelled by distillation from the reaction part for the synthesis of lactide at any of the steps mentioned above contains lactic acid monomer, lactic acid dimer (lactoyllactic acid), acrylic acid, pyruvic acid and water as impurities. Of these impurities, the acid impurities inconvenience the severance of the polylactic acid chains and the consequent production of high molecular weight polylactic acid by ring opening polymerization.
The impurities are generally removed from crude lactide by methods such as crystallization; extraction or distillation to permit the production of purified lactide. When the separation of lactide from these impurities is effected by crystallization, however, the yield by the crystallization is too low to be commercially acceptable for the production of lactide of high purity. In the separation by distillation, lactic acid and lactide undergo thermal polymerization, hydrolysis and such reactions. Since their thorough separation is difficult and the yield of lactide is low, this method is not acceptable.
The purification of lactide was also performed in a mixture of dual solvent such as acetone and water and claimed to produce lactide of high purity. This method suffers from heavy loss due to the crystallization because the solubility of lactide in acetone is high.
When L-lactic acid is used as the raw material for the crude lactide, the lactic acid undergoes racemization and gives rise to meso-lactide and a minute amount of D-lactide in addition to L-lactide, because high reaction temperature and long retention time must be used generally for allowing the reaction to proceed.
U.S. Pat. No. 5,502,215 titled “Method for purification of lactide” discloses washing of crude (L/D) lactide with cold water at 20° C. to remove/control the water soluble impurities such as meso-lactide and lactic acid monomer. Further, the lactide crystals are recrystallised from boiling acetone.
U.S. Pat. No. 5,053,485 titled “Polymer lactide, method for preparing it and a composition containing it” describe the purification of crude DL-lactide by using inexpensive solvents, such as toluene, benzene and diethyl ether.
U.S. Pat. No. 5,136,057 claims a process for preparing lactide by depolymerization of a polylactide. The depolymerised product is scrubbed with a solvent such as acetone, to form a solution of lactide and its lactic acid value impurities. The solution is further concentrated to start precipitation of lactide, and diluted with water, preferably water cooled to 0°-5° C., in an amount sufficient to precipitate the lactide substantially, leaving the lactic acid values in the aqueous acetone solution. Lactide, substantially free of its impurities, is separated, and purified further, by-washing, drying and recrystallization from non-reactive solvents, e.g., toluene.50% yield in cycle 1 with yields increasing to 70 and 84% in cycles 2 and 3.
U.S. Pat. No. 5,543,494 titled “Process for the production of poly (lactic acid)” discloses purification of L-lactide by boiling with anhydrous toluene and also successive washings with anhydrous cold toluene is disclosed yielding 60% of L-lactide.
Further, for the purpose of synthesizing polylactic acid with a high molecular weight and having a high optical purity, it is necessary that the lactic acids which are components of the lactide being used as the raw material posses a high optical purity, namely the lactide itself should possess a high optical purity.
Thus there is no process in the art for the preparation of L-lactide which could give the product in high yield and having an optical purity of 100%. Further, it is observed that yield and optical purity of L-lactide is sensitive to temperature, pressure, catalyst used etc. Also, preparation by batch wise-operating process leads to deterioration in the overall-yield and to a reduction in optical purity.